Shorter answer:
"Chiptune" means 'tunes generated by embedded sound chips' in my circles. I've never seen anyone apply a bit standard to that, before but there certainly are categorical differences in kinds. 

* "Generated sound" music: 8-bit music is generally synthesized from a few basic component waveforms and mixed. 
* "Tracked" or "Tracker" music: 16-bit generally runs a program like a Music Demo playing MODs. That is, it's synthesized on the fly from stored waveforms. Other names for this sort of thing are: Demo, MODs, instrumental synth, and so on. However, to have a meaningful sample, the sample must be recorded in some manner. Since there are multiple forms of doing *that*, (e.g. PCM, FM, etc) there are actually multiple terms that can apply. Even with this niftier hardware, some used the hardware as above, generating and mixing waves.
* "Streaming" music: 32+bit systems generally pipe in audio streams and don't synthesize anything except, sometimes, sound effects. Ironically, that means that in significant ways, modern gaming audio processing is less advanced than earlier systems. I find that odd.

Long and involved nerd answer:

Early sound chips generated their sounds by adjusting the frequency and amplitude of a few basic waveforms. These were generated by simple signal generation circuits and the chips that compiled these functions together are known as programmable sound generators (PSG). What I've seen pretty much always uses some combination of sine, (rarely) cosine, triangle, square, saw, and something noisey. (https://en.wikipedia.org/wiki/Sound_chip)

The home consoles were never MIDI. (fun fact: MIDI predates the NES by a year or two) A MIDI song is a set of commands for playing notes that are sent to various instruments. The recipient of those commands is supposed to play them. The results can be anything from terrible synthetic music to indistinguishable from a recording of a natural instrument or even an *actual instrument*. MIDI is a control language optimized for *making music*, **not** an audio format. 

Since MS Windows 3.11's Multimedia extensions, it's been normal for MS to package in some sort of MIDI synthesizer. (which improved over time and then fell back to poor in either XP or 7. Not sure what support looks like in 8 and 10) This sets many opinions as to what MIDI can do, sadly.

Cheap MIDI synth. sounds cheap and be quite reminiscent of older styles of synthesis. (Some soundfonts exist specifically to narrow that gap even further.) High quality MIDI synth. can sound like a CD recording of an orchestra. If you think MIDI music *has* to suck, it's because you haven't heard MIDI used to direct good instrumentation and don't understand its potential.

http://www.iub.edu/~emusic/361/midi.htm

https://www.midi.org/

Amiga had a much more powerful sound chip than most systems of its era. (graphics, too, as it happens) One of the results of that was SoundTracker and the MOD format. Tracker music is a sequence of notes and effects on those notes generated from samples transformed and mixed. The notation part of the format of tracker music is actually quite like MIDI, but it carries along its desired samples and rules for using them. This makes it harder to upgrade but more likely to sound right.

https://en.wikipedia.org/wiki/Module_file

Related, but aside:

https://en.wikipedia.org/wiki/Audio_file_format

https://en.wikipedia.org/wiki/MOS_Technology_SID